A circuit board structure is widely applied in various electronic products, such as consumer electronic products. Because the consumer electronic products are getting more and more compact, the circuit board structure has to shrink its dimensions as well. This means that pitch, the sum of the line width d1 and the spacing d2 as shown in FIG. 2A, must be reduced so as to integrate more elements/devices on a more compact circuit board structure.
FIG. 1 illustrates a conventional circuit board structure. FIG. 2A and FIG. 2B are enlarged views of FIG. 1 illustrating formation of bump pads using a semi additive process. As shown in FIG. 1, the circuit board structure 1 includes an intermediate substrate 10 having interconnections 40 therein and circuit patterns (20, 30) on both upper and lower surfaces. The interconnections 40 electrically connect the upper and lower circuit patterns (20, 30). The circuit board structure 1 further includes an upper dielectric layer 50 overlying the upper circuit patterns 20. The upper dielectric layer 50 has a plurality of vias 70 for electrically connecting the upper circuit patterns 20 and an external circuit (not shown). The circuit board structure 1 further includes a lower dielectric layer 60 covering the lower circuit patterns 30. The lower dielectric layer 60 has a plurality of vias 80 for electrically connecting the lower circuit patterns 30 and the external circuit (not shown). The circuit board structure 1 further includes an upper solder mask layer 90 and bump pads 105 overlying the upper dielectric layer 50, and further includes a lower solder mask layer 100 covering the lower dielectric layer 60. The upper solder mask layer 90 and lower solder mask layer 100 respectively expose bump pads 105 and vias 80 for subsequent surface finishing process, e.g. electroless nickel electroless palladium immersion gold (ENEPIG) process. Referring to FIG. 2A, prior to the ENEPIG process, a so-called semi additive process (SAP) is used to form copper wires 13 with a spacing d2, for example, of 30 μm between each other. Referring to FIG. 2B, after the ENEPIG process, bump pads 105 are made of nickel layer 14, palladium layer 15 and gold layer 16 conformally overlying the copper wires 13. The bump space d3 between adjacent bump pads 105 is reduced and less than 20 μm so that a short circuit might occur.
Accordingly, a circuit board structure capable of solving the aforementioned drawbacks is desirable.